Much attention has been directed to the development of packaging materials in a film, a semi-rigid or rigid sheet and a rigid container made of a thermoplastic composition. In such applications, the polymeric composition preferably acts as a barrier to the passage of a variety of permeant compositions to prevent contact between ,e.g., the contents of a package and the permeant. Improving barrier properties is an important goal for manufacturers of film and thermoplastic resins.
Barrier properties arise from both the structure and the composition of the material. The order of the structure (i.e.,), the crystallinity or the amorphous nature of the material, the existence of layers or coatings can affect barrier properties. The barrier property of many materials can be increased by using liquid crystal or self-ordering molecular technology, by axially orienting materials such as an ethylene vinyl alcohol film, or by biaxially orienting nylon films and by using other useful structures. Internal polymeric structure can be crystallized or ordered in a way to increase the resistance to permeation of a permeant. A material can be selected, for the thermoplastic or packaging coating, which prevents absorption of a permeant onto the barrier surface. The material can also be selected to prevent the transport of the permeant through the barrier. Permeation that corresponds to Fick's law and non-Fickian diffusion has been observed. Generally, permeation is concentration and temperature dependent regarding mode of transport.
The permeation process can be described as a multistep event. First, collision of the permeant molecule with the polymer is followed by sorption into the polymer. Next, migration through the polymer matrix by random hops occurs and finally the desorption of the penetrant from the polymer completes the process. The process occurs to eliminate an existing chemical concentration difference between the outside of the film and the inside of the package. Permeability of an organic molecule through a packaging film consists of two component parts, the diffusion rate and solubility of the molecule in the film. The diffusion rate measures how fast molecule transport occurs through the film. It affects the ease with which a permeant molecule moves within a polymer. Solubility is a measure of the concentration of the permeant molecule that will be in position to migrate through the film. Diffusion and solubility are important measurements of a barrier film's performance. There are two types of mechanisms of mass transfer for organic vapors permeating through packaging films: capillary flow and activated diffusion. Capillary flow involves small molecules permeating through pinholes or highly porous media. This is of course an undesirable feature in a high barrier film. The second, called activated diffusion, consists of solubilization of the penetrants into an effectively non-porous film at the inflow surface, diffusion through the film under a concentration gradient (high concentration to low concentration), and release from the outflow surface at a lower concentration. In non-porous polymeric films, therefore, the mass transport of a penetrant includes three steps -- sorption, diffusion, and desorption. Sorption and desorption depend upon the solubility of the penetrant in the film. The process of sorption of a vapor by a polymer can be considered to involve two stages: condensation of the vapor onto the polymer followed by solution of the condensed vapor into the polymer. For a thin-film polymer, permeation is the flow of a substance through a film under a permeant concentration gradient. The driving force for permeation is given as the pressure difference of the permeant across the film. Several factors determine the ability of a permeant molecule to permeate through a membrane: size, shape, and chemical nature of the permeant, physical and chemical properties of the polymer, and interactions between the permeant and the polymer.
A permeant for this application means a material that can exist in the atmosphere at a substantial detectable concentration and can be transmitted through a known polymer material. A large variety of permeants are known. Such permeants include water vapor, aromatic and aliphatic hydrocarbons, monomer compositions and residues, off odors, off flavors, perfumes, smoke, pesticides, toxic materials, etc. A typical barrier material comprises a single layer of polymer, a two layer coextruded or laminated polymer film, a coated monolayer, bilayer or multilayer film having one or more coatings on a surface or both surfaces of the film or sheet.
The two most widely used barrier polymers for food packaging are ethylene-vinyl alcohol copolymers (EVOH) ethylene vinyl acetate copolymers (EVA) and polyvinylidene chloride (PVDC). Other useful thermoplastics include ethylene acrylic materials including ethylene acrylic acid, ethylene methacrylic acid, etc. Such polymers are available commercially and offer some resistance to permeation of gases, flavors, aromas, solvents and most chemicals. PVDC is also an excellent barrier to moisture while EVOH offers very good processability and permits substantial use of regrind materials. EVOH copolymer resins are commonly used in a wide variety of grades having varying ethylene concentrations. As the ethylene content is reduced, the barrier properties to gases, flavors and solvents increase. EVOH resins are commonly used in coextrusions with polyolefins, nylon or polyethylene terephthalate (PET) as a structural layer. Commercially, amorphous nylon resins are being promoted for monolayer bottles and films. Moderate barrier polymer materials such as monolayer polyethylene terephthalate, polymethyl pentene or polyvinyl chloride films are available.
Substantial attention is now directed to a variety of technologies for the improvement of barrier properties. The use of both physical barriers and active chemical barriers or traps in packaging materials are under active investigation. In particular, attention has focused on use of specific copolymer and terpolymer materials, the use of specific polymer alloys, the use of improved coatings for barrier material such as silica metals, organometallics, and other strategies.
Another important barrier technology involves the use of oxygen absorbers or scavengers that are used in polymeric coatings or in bulk polymer materials. Metallic reducing agents such as ferrous compounds, powdered oxide or metallic platinum can be incorporated into barrier systems. These systems scavenge oxygen by converting it into a stable oxide within the film. Non-metallic oxygen scavengers have also been developed and are intended to alleviate problems associated with metal or metallic tastes or odors. Such systems include compounds including ascorbic acid (vitamin C) and salts. A recent introduction involves organometallic molecules that have a natural affinity for oxygen. Such molecules absorb oxygen molecules into the interior polymer chemical structure removing oxygen from the internal or enclosed space of packaging materials.
Packaging scientists are continuing to develop new polymeric films, coated films, polymeric alloys, etc. using blends of materials to attain higher barrier properties. Many of these systems have attained some degree of utility but have failed to achieve substantial commercial success due to a variety of factors including obtaining barrier performance at low cost.
One problem that arises when searching for polymer blends or compounded polymeric materials, relates to the physical properties of the film. Films must retain substantial clarity, tensile strength, resistance to penetration, tear resistance, etc. to remain useful in packaging materials. Blending unlike materials into a thermoplastic before film extrusion often results in a substantial reduction of film properties. Finding compatible polymer materials for polymer alloys, and compatible additives for polymeric materials typically require empirical demonstration of compatibility and does not follow a clearly developed theory. However compatibility can be demonstrated by showing that the compounded material obtains an improved barrier quality with little reduction in clarity, processability, or structural properties using conventional test methods. Accordingly, a substantial need exists for development of materials that can be incorporated into polymeric material to form a packaging thermoplastic having excellent barrier properties without any substantial reduction in structural properties.